1. Field of the Invention
The present invention relates to an energy trap type piezoelectric filter device using a second harmonic wave of a thickness-extension vibration mode. In particular, the present invention relates to a piezoelectric filter device having a structure including two piezoelectric filters, which are electrically connected to each other via a relay capacitor.
2. Description of the Related Art
FIG. 10 is a sectional view illustrating a conventional energy trap type piezoelectric filter device using a second harmonic wave of a thickness-extension vibration mode.
As shown in FIG. 10, a piezoelectric filter 51 including a piezoelectric substrate plate 52, which is polarized in its thickness direction, is provided. A ground electrode 53 is disposed in the center of the piezoelectric substrate plate 52. On the upper surface of the piezoelectric substrate plate 52, an input electrode 54 is arranged opposite to a portion of the ground electrode 53 with the piezoelectric substrate plate being partially interposed therebetween. On the bottom surface of the piezoelectric substrate plate 52, an output electrode 55 is provided. Similarly, the output electrode 55 is arranged opposite to a portion of the ground electrode 53 with the piezoelectric substrate plate being partially interposed therebetween. Further, the output electrode 55 is provided in a position so as not to overlap the input electrode 54 through the piezoelectric substrate plate 52.
Using the above structure, as soon as an input signal is applied between the input electrode 54 and the ground electrode 53, the piezoelectric filter 51 functions as a double mode filter that operates by virtue of a second harmonic wave of a thickness-extension vibration, thus producing an output signal through the output electrode 55.
Another conventional piezoelectric filter device includes two piezoelectric filters 51, and the two piezoelectric filters 51 are electrically connected to the ground potential via a relay capacitor.
However, in the above described conventional piezoelectric filter device, when the two piezoelectric filters 51 are electrically connected with each other through a relay capacitor, the height of the ground electrode 53 of one piezoelectric filter 51 is different from that of another ground electrode. As a result, it is difficult for the piezoelectric filter device to obtain consistent characteristics.
In the above piezoelectric filter 51, the ground electrode 53 is located at a central position in the thickness direction of the piezoelectric substrate plate 52. The piezoelectric filter 51 is usually obtained by cutting a mother piezoelectric substrate plate. However, in a mother piezoelectric substrate plate, it is impossible to avoid warpage and waviness.
Nevertheless, when a piezoelectric filter 51 produced via this method is tested, the height of the ground electrode 53 differs from one piezoelectric filter to another. As a result, in the above described conventional piezoelectric filter device in which the two piezoelectric filters 51 are electrically connected with each other via a relay capacity, the height of the ground electrode 53 of one piezoelectric filter 51 differs from that of another ground electrode. Accordingly, it is difficult to obtain consistent characteristics with this piezoelectric filter device.
In order to overcome the above-described problems, preferred embodiments of the present invention provide a piezoelectric filter device, which is free from the above-described defects existing in the above-described prior art. More particularly, preferred embodiments of the present invention provide an improved piezoelectric filter device which includes two energy trap type piezoelectric filters each having an ground electrode disposed within a piezoelectric substrate plate and operating using a second harmonic wave of a thickness-extension vibration, thereby producing an improved piezoelectric filter device having stabilized filter characteristics.
According to preferred embodiments of the present invention, a piezoelectric filter device operates using a second harmonic wave of a thickness-extension vibration mode and includes a first piezoelectric filter and a second piezoelectric filter which are electrically connected to each other with a relay capacitor interposed therebetween, the first and second piezoelectric filters are polarized in the thickness directions thereof and include a pair of mutually opposing piezoelectric substrate plates each having a first main surface and a second main surface, a ground electrode provided within each of the piezoelectric substrate plates, an input electrode provided on the first main surface of each piezoelectric substrate plate such that the input electrode is opposite to one portion of the ground electrode with the piezoelectric substrate plate being partially interposed therebetween, an output electrode provided on the second main surface of each piezoelectric substrate plate such that the output electrode is opposite to the ground electrode except in the one portion thereof, with the piezoelectric substrate plate being partially interposed therebetween.
When a distance between the first main surface of the first piezoelectric filter and the adjacent ground electrode is ta1, a distance between the second main surface of the first filter and the adjacent ground electrode is tb1, a distance between the second main surface of the second piezoelectric filter and the adjacent ground electrode is ta2, a distance between the first main surface of the second piezoelectric filter and the adjacent ground electrode is tb2, the thicknesses of the piezoelectric substrate plates of the first and second piezoelectric filters are T1 and T2 respectively, tb1xe2x88x92ta1 is 0 to 0.1 T1, Tb2xe2x88x92ta2 is 0 to 0.1 T2, alternatively, tb1xe2x88x92ta1 is xe2x88x920.1 T1 to 0, Tb2xe2x88x92ta2 is 0.1 T2 to 0.
Preferably, the thicknesses T1 and T2 of the piezoelectric substrate plates of the first and second piezoelectric filters are substantially equal to each other.
More preferably, when the thicknesses T1 and T2 of the piezoelectric substrate plates of the first and second piezoelectric filters are substantially equal to each other, and when the thicknesses are T, in a perpendicular coordinate system including tb1xe2x88x92ta1 and tb2xe2x88x92ta2, the value of tb1xe2x88x92ta1 and the value of tb2xe2x88x92ta2 will fall within either a range A or a range B as shown in FIG. 1.
Further, in the piezoelectric filter device according to preferred embodiments of the present invention, the first and second piezoelectric filters may either be formed separately, or they can be laminated together to define an integral body in a manner such that the vibrations of the piezoelectric vibration sections thereof are not suppressed.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments hereof with reference to the attached drawings.